64-Bit CPUs: AMD Hammer vs. Intel IA-64

The most noticeable enhancement to Hammer is its 64-bit register file (we’ll get to the 64-bit instructions in a minute). All the old familiar x86 registers are extended to 64 bits and new registers added, with RISC-like names R8 through R15. Obviously, existing x86 binaries won’t see the upper half of the eight new 64-bit registers, or the eight new registers at all. The enhancements are visible to new 64-bit code only.

AMD’s sixteen 64-bit registers are a far cry from Intel’s 128 general-purpose plus 128 floating-point registers. Even in its 64-bit mode, AMD has one-sixteenth the quantity of registers that IA-64 has. There’s no argument that more registers is better, although there’s plenty of contention over how much better. How many registers is enough? There comes a point of diminishing returns, but we’d bet that most programmers and compiler writers would prefer some number greater than 16.

If it makes AMD fans feel any better, Itanium’s register file is so big it takes two clock cycles to access a register, adding a stage to the pipeline. If it makes Intel fans feel any better, that delay’s probably going to go away in McKinley and future IA-64 processors.

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